The invention relates to a detector for use in the alignment of a laser beam. In particular, the invention pertains to the use of an accurate and low cost thermal detector for aligning a high power laser beam.
In the industrial application of lasers it is sometimes necessary to transport the laser beam over significant distances, frequently many feet. Laser beam transport may be accomplished by deflecting the beam off several mirrors or other optical elements which are suitably aligned along a desired beam path. In order to perform these functions, one must properly align the laser beam with each of the optical elements. If the laser beam is visible, alignment of each optical element can be performed by observing the beam position on a target placed downstream of the optical element and adjusting the element until the visible beam is on target. In some cases, alignment of a laser beam whose radiation is not visible has been accomplished by using a visible laser beam, typically a Helium-Neon (HeNe) laser, as a tracking beam. In such cases, the HeNe laser beam is first aligned with the main laser beam using techniques that are known to those skilled in the art. Thereafter, the optical elements may be aligned using the visible tracking beam and the targeting technique described above.
In the development of a laser beam transport system for use in the laser welding of repair sleeves in defective steam generator heat exchanger tubes, it is necessary to have a detector system for monitoring the position of a high power CO.sub.2 laser beam. Many different detectors have been developed that can be used to aid in the alignment of such laser beams. These detectors include photosensitive devices, thermal devices, pyroelectric devices and others. Although detectors respond to a wide range of laser wavelengths and power levels, they are not well suited for high power applications. Some devices consist of a single sensing element and thus are not adequate to allow the laser beam to be aligned with sufficient accuracy. Other detectors consist of linear or two dimensional arrays of photosensitive sensing elements, and may require complex resolution circuitry. Photosensitive detectors also tend to be fragile and they do not respond well to the high laser powers encountered in the application contemplated. Furthermore, they may not be configured in the shapes and sizes required for the application.